1. Field of the Invention
This invention relates to a method for removing dimethyl sulfide (DMS) and tertiary butyl mercaptan (TBM) as the sulfur compounds present in city gas.
2. Related Art of the Invention
The sulfur compounds must be removed from the city gas before it is purged into the air, while a city gas system is under construction or repair. The city gas as the fuel for fuel cells must be desulfurized, to prevent the deactivation of steam reforming catalysts, such as those on Ru and Ni.
Activated carbon, either as it is or modified by some reagent, has been mainly used for removal of the sulfur compounds from the fuel gas.
Desulfurization under consideration for the city gas as the fuel for fuel cells involves hydrodesulfurization of the sulfur compounds into hydrogen sulfide, which is then removed by zinc oxide.
Removal of the sulfur compounds in air for deodorization is generally effected by oxidative decomposition of mercaptan compounds at room temperature in the presence of an oxide of transition metal, e.g., copper or manganese, or by adsorption of these compounds on activated carbon, either as it is or supported.
Each of the above conventional techniques has its own disadvantages.
Activated carbon is difficult to regenerate, because of its combustibility, and must be replaced when sufficiently deactivated. When modified by some reagent, it removes sulfur compounds by chemical reaction, which makes it difficult to regenerate, although showing excellent adsorption-related characteristics (the first problem).
A combination of hydrodesulfurization pretreatment and removal of H2S in the presence of zinc oxide for fuel cell fuels needs high temperature of 300 to 400xc2x0 C. for the hydrodesulfurization process, which lowers overall efficiency of the power output by the cell. It is also necessary to replace deactivated zinc oxide (the second problem).
The deodorization in an oxidative atmosphere in the presence of an oxide of transition metal, e.g., copper or manganese, mercaptan compounds may be partially oxidized at room temperature into a disulfide, which will cause offensive odor, and is difficult to remove the sulfur compounds, e.g., sulfides, other than mercaptan compounds (the third problem).
In consideration of the above first and second problems involved in the conventional techniques for removing the sulfur compounds, it is an object of the present invention to provide a method for removing dimethyl sulfide (DMS) and/or tertiary butyl mercaptan (TBM) as the sulfur compounds present in city gas. It is another object of the present invention to provide a method for removing the sulfur compounds by the aid of the above adsorbent.
The present invention provides a method for removing dimethyl sulfide (DMS) and/or tertiary butyl mercaptan (TBM) as the sulfur compounds present in city gas by using the sulfur compounds adsorbent containing one of faujasite, xcex2, L and MFI type zeolite, and also provides a method for removing the sulfur compounds, in which the above adsorbent is intermittently regenerated.
One aspect of the present invention is a method for removing dimethyl sulfide (DMS) and/or tertiary butyl mercaptan (TBM) as sulfur compounds present in city gas by using a sulfur compound adsorbent containing one of faujasite, xcex2, L and MFI type zeolite.
Another aspect of the present invention is a method for removing sulfur compounds, wherein said zeolite comprises Si and another type of metal M in the framework, Si/M atomic ratio being 250 or less and M being selected from the group consisting of Al, Fe and Ga.
Still another aspect of the present invention is a method for removing sulfur compounds, wherein the cation in said zeolite is H+.
Yet another aspect of the present invention is a method for removing sulfur compounds, wherein said zeolite is treated for dealuminization.
Still yet another aspect of the present invention is a method for removing sulfur compounds, wherein said sulfur compound adsorbent contains an inorganic binder.
A further aspect of the present invention is a method for removing sulfur compounds, wherein said inorganic binder is silica.
A still another aspect of the present invention is a method for regenerating an adsorbent for sulfur compounds, containing one of faujasite, xcex2, L and MFI type zeolite and an inorganic binder, wherein at least heating step is included for regenerating said adsorbent after said adsorbent is used for a process which removes dimethyl sulfide (DMS) and/or tertiary butyl mercaptan (TBM) as the sulfur compounds present in city gas.
Yet another aspect of the present invention is the method for regenerating an adsorbent for sulfur compounds, wherein a gas released from said adsorbent during said regeneration process is oxidized in the presence of an oxidation catalyst.
Yet another aspect of the present invention is a method for removing sulfur compounds, comprising a first step which passes city gas which contains dimethyl sulfide (DMS) and/or tertiary butyl mercaptan (TBM) as the sulfur compounds over a first adsorbent containing ultrastable Y type zeolite, and second step which passes the gas from the first step over a second adsorbent containing MFI or faujasite type zeolite having an alkaline or alkaline-earth metal in the cation site.
Yet another aspect of the present invention is a method for removing sulfur compounds, comprising:
a first step of passing a mercaptan-containing city gas over a first adsorbent containing zeolite with H+ as the cation or dealuminized zeolite; and
a second step of passing the gas from the first adsorbent over a second adsorbent containing zeolite having an alkaline or alkaline-earth metal in the ion-exchanged site.
City gas is intentionally incorporated with trace quantities of a sulfur compound as the odorant in order to give a warning of gas leak. The odorant type differs by city gas supplier. In Japan, tertiary butyl mercapatan (TBM), dimethyl sulfide (DMS) or tetrahydrothiophene is added to city gas to several ppm. Unlike natural gas or coke oven gas which contains various types of sulfur compounds, the city gas is intentionally incorporated with the sulfur compound of known structure at a known concentration. Therefore, the sulfur compound in the city gas can be removed by adsorption, when an adequate adsorbent is selected to efficiently adsorb the compound.
The present invention removes the sulfur compounds by the aid of an adsorbent which contains one of faujasite (X or Y type), xcex2, L and MFI type zeolite. It is composed of inorganic materials, and can be regenerated under heating.
Tertiary butyl mercaptan and dimethyl sulfide are the typical odorants used for the city gas, the latter being more difficult to remove by adsorption and showing an earlier break-through. It is therefore important to increase amount of dimethyl sulfide adsorbed. Use of faujasite, xcex2, L or MFI type zeolite can increase the adsorbed quantity from that adsorbed by other types of zeolite, such as A type.
The present invention needs no heating for adsorption, because adsorbed amount increases as temperature decreases, and is more advantageous over the method involving hydrodesulfurization, because of simplified equipment designs and saved energy.
The adsorbent of the present invention comprises Si and another type of metal M in the skeleton structure. Zeolite having an Si/M molar ratio of 250 or less shows excellent adsorption characteristics, which tend to improve as Si/M ratio decreases. It is known that zeolite of low Si/M molar ratio is generally high in acidity. The inventors of the present invention have investigated zeolite acidity by the temperature-programmed desorption method of ammonia (ammonia TPD), to find that zeolite of low Si/M molar ratio shows a larger quantity of ammonia desorbed, or higher in acidity, indicating that acidity of zeolite contributes to removal of a sulfur compound present in fuel gas. It is preferable that an element M that comprises, together with Si, the zeolite framework for the present invention is Al, Fe or Ga. This combination gives higher characteristics of removing sulfur compounds than any other combination.
The zeolite contains an ion-exchangeable cation, and shows excellent characteristics for removing the sulfur compound from fuel gas, when the cation is H+, conceivably by virtue of increased number of strong acid sites. It is therefore considered that zeolite having strong acid sites adsorbs sulfur compounds well.
It is preferable that the zeolite for the present invention is treated for dealuminization, which is considered to increase the number of strong acid sites. Any known dealuminization method can be used for the present invention, including hydrothermal treatment, heating in an acidic solution, or heating in a gaseous atmosphere containing a silicon compound, e.g., silicon tetrachloride.
TBM breaks through in a relatively short time in the absence of DBM, when passed over zeolite containing H+ as the cation or dealuminized zeolite. However, the period before it breaks through will be extended in the presence of DMS. TBM turns into an isobutene-like species when adsorbed on the acid site, and it is considered that polymerization of the isobutene-like species is prevented in the presence of DMS to control poisoning of the acid sites as the active sites.
It is preferable that the adsorbent of the present invention is in the form of pellet or the like, and that the binder is inorganic. Use of an organic binder, such as a cellulosic one, may deteriorate adsorbent strength, because of its possible combustion during the regeneration under heating. This problem can be avoided when the adsorbent is regenerated at a temperature level below the combustion point of the organic binder in question. However, this tends to greatly limit the working temperature range, knowing that a cellulosic compound starts to decrease in strength at 160xc2x0 C. or more, and the regeneration is frequently insufficient, because of a temperature distribution within the adsorbent bed during the regeneration under heating. The inorganic binders useful for the present invention include fired colloids, e.g., silica, alumina and titania sol; clay-based compounds; and cement and gypsum. Of these, more preferable one is silica, dehydrated by firing silica sol. Fired alumina sol, clay-based compounds and alumina cement may show a lower strength, whereas Li silicate and water glass, although giving a high form strength, will show lower adsorption characteristics. Use of silica as the inorganic binder gives a strong form without sacrificing its adsorption characteristics.
It is preferable that the adsorbent of a related invention of the present invention contains copper oxide, manganese oxide, a compound oxide of copper and manganese, or platinum group element. Each of these compounds or element has activity of partially oxidizing a mercaptan compound, and hence can greatly improve characteristics of the adsorbent for removing a mercaptan compound in the presence of oxygen. This related invention of the present invention can solve the above mentioned third problem. A platinum group element can exhibit the above characteristics sufficiently in only a small quantity, when finely dispersed in a porous carrier, such as alumina. A disulfide will be formed as the partial oxidation product in the presence of the above compound or element, but release of the sulfur compound can be well controlled by selecting a zeolite of high disulfide adsorption capacity.
Furthermore, it is preferable to regenerate the deactivated adsorbent of the present invention, which allows to use the adsorbent for extended periods. The adsorbent can be regenerated by various methods, such as passing a sulfur-free gas over the adsorbent, heating the adsorbent, and treating the adsorbent under a vacuum. The method involving at least heating is preferable, because it completes the regeneration faster.
Furthermore, it is preferable to oxidize, in the presence of an oxidation catalyst, the gas released out of the adsorbent during the regeneration process. This is to facilitate the post-treatment step, e.g., absorption of the sulfur compound by an alkaline aqueous solution, by converting it into sulfur dioxide as an acidic gas. This prevents release of odorous sulfur dioxide and also protects the equipment from corrosion, although periodic exchange of the treatment solution is required.
It is preferable that the oxidation catalyst for the present invention contains platinum, which is more resistant to poisoning by sulfur than other precious elements, e.g., palladium and rhodium, and sustains oxidation activity for more extended periods.
Ultrastable Y type (USY) is one of the most active zeolite types for removing an odorant from city gas. However, USY needs a long time before the stable adsorption band is formed in the adsorbent layer, tending to show declined DMS adsorption characteristics in a short time after starting adsorption and then high adsorption characteristics thereafter. Tendency of having strong acid sites is considered to be responsible for such behavior of USY.
By contrast, faujasite (Y or X) type zeolite having an alkaline or alkaline-earth element in the ion-exchanging site, although showing high adsorption characteristics during the initial stage, adsorbs smaller quantities of sulfur compounds, rapidly losing its adsorption ability when the break-through starts, increasing sulfur content of the gas which has passed over it. MFI type zeolite, on the other hand, has high DMS adsorption capacity but lower TBM adsorption capacity than USY.
The present invention provides a method for removing sulfur compounds from fuel gas, comprising a first step which passes a sulfur-containing fuel gas over a first adsorbent containing ultrastable Y (USY) type zeolite, and second step which passes the gas from the first step over a second adsorbent containing MFI or faujasite type zeolite having an alkaline or alkaline-earth metal in the ion-exchanging site. During the initial stage of the adsorption process before the first adsorbent exhibits high adsorption characteristics, the sulfur compound breaking through the first step is mainly removed by the second step downstream of the first step by the second adsorbent, which contains MFI type zeolite capable of adsorbing DMS difficult to remove by the first adsorbent, or faujasite zeolite having an alkaline or alkaline-earth element in the ion-exchanging site, showing high adsorption characteristics from the very start of the adsorption process. A combination of these steps secures high desulfurization capacity as a whole for extended periods, because of stabilized adsorption capacity of the first adsorbent in a certain time after start of the adsorption process, although the second adsorbent adsorbs smaller quantities of sulfur compounds than USY, as described above.
Characteristics of zeolite for removing sulfur compounds depend on its acidity. However, when fuel gas is passed over zeolite having strong acidic sites for extended periods, trace quantities of hydrogen sulfide may be detected in the exhaust gas, depending on sulfur compound type present in the feed gas. This problem can be solved, when zeolite having a high acidity is followed by an adsorbent having a high capacity of adsorbing hydrogen sulfide if high desulfurization capacity is required. Adsorbent of high H2S adsorption capacity include faujasite type zeolite with an alkaline metal as the cation.